In water year 2007, the U.S. Geological Survey, in cooperation with the National Park Service, began a 2-year study to collect hydrologic, water-quality, and biological data to provide a baseline characterization of the North Fork Flathead River from the United States-Canada border to its confluence with the Middle Fork of the Flathead River near Columbia Falls, Montana. Although mining in the Canadian portion of the North Fork Basin was banned in 2010 by a Memorandum of Understanding issued by the Province of British Columbia, baseline characterization was deemed important for the evaluation of any potential future changes in hydrology, water quality, or aquatic biology in the basin. The North Fork Basin above Columbia Falls (including Canada) drains an area of 1,564 square miles, and the study area encompasses the portion of the basin in Montana, which is 1,126 square miles. Seasonal patterns in the hydrology of the North Fork are dominated by the accumulation and melting of seasonal snowpack in the basin. Low-flow conditions occurred during the late-summer, fall, and winter months, and high-flow conditions coincided with the spring snowmelt. Substantial gains in streamflow occurred along the study reach of the North Fork, 85 percent of which were accounted for by tributary inflows during low-flow conditions, indicating unmeasured streamflow inputs along the main stem were 15 percent or less.